Warnings: Undoubtedly AU sometime in the near future, Spoilers for the Hueco Mundo Arc and beyond

AN: For the Bleach contest on LJ. The prompt was "Reaction."

Enthalpy of formation: the heat released or absorbed in a chemical reaction at constant pressure when simple substances combine into a more complex one.

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He knows with every breath he takes that he will die here. In this place. His enemy is stronger, and he knows it. Feels it with every fiber of himself. Understands that this will cost him his life, a Pyrrhic victory. But the alternative is unthinkable. That this abomination should survive to harm her further. To destroy what Byakuya is now willing to give his life to protect.

This will be his end. His last resting place. His grave. And a part of him is glad for it. Glad to kill what he has allowed himself to become. A man so weak that he would value an already broken promise more than a life. He's made so many mistakes the last few years. So many things he wishes that he could take back. But never her, never bringing her into his home. He could never regret that.

It is funny almost. The clarity that his imminent mortality brings. The picture perfect view of his own emotions and actions. Of how horribly he has treated her. Slighted her in both word and deed though never in thought. Did not defend her to her naysayers and tormentors. Did not shield her from the execution order. Always coming too late to the fight to prevent injury. To stop her blood from spilling to the white sands.

Even then, even after he told her the truth, he did not say all of it. Did not say that Hisana's promise is not the only reason he watches after her. Did not speak the guarded secret of his soul. That he values her more than his birth family or the priceless heirloom he wears. More than his position as captain and head of his clan. More even than he ever did Hisana.

He finds his feet again and shoves her behind him. Moves her out of the way as carefully as he can but with enough force to keep her out of the fight. She goes reluctantly but can do little else before she turns to check on her companions, who are injured but alive. She knows just as he does that this opponent is beyond her. She can hardly stand, let alone walk as she almost crawls away. Byakuya fears that she won't get far.

The Espada is huge. Enormous. Massive and bulky in a way that not even Komamura's bankai can manage. And the number zero stares out at him from its place on the Arrancar's shoulder. Black as the sky here and all the more deadly for its simplistic shape and meaning. A promise of agony and death on the endless sands of this godforsaken place. Of her voice shrieking his name just as she screamed after his lieutenant and her human nakama.

The first strike is lightening swift. A fist roars through the air towards him. He flashes away instantly but is still nearly caught by the edge of a finger. It is all he can do not to shudder at the ferocity of the strike, at the speed that the Arrancar repositions and lashes out again. His movements are rough and not at all graceful, not like a predator on the prowl but more similar to a human-shaped tsunami. Violent and overflowing with energy but the more dangerous for the unexpected angles and sharp barbs jutting out beneath the torrent.

Even Renji and the Kurosaki brat have a sense of style, a blade to actually defend against. This monster is all pure, unadulterated power and brute force. Blow after blow directed his way without even enough time to counter, just barely enough to keep his head and body attached and in working order. Senbonzakura, though elegant and fierce, is no match for his sheer strength. A sword – a mere knife – brought to a missile fight. A haze of pink petals that are scarcely enough to scratch his thick skin.

He laughs at that. This Espada laughs and mockingly grabs a fist full of them as they become true swords. But it is like a stick pin to an elephant, and he crushes the blades in the palm of his hand and scatters them like dust. Ineffective and now broken.

Kidoh would seemingly be an ideal weapon in this instant, but he barely has time to think before the next strike comes. Let alone decide a spell and employ its use. Kidoh requires concentration, and his mind simply isn't up to the task. He can scarcely focus beyond the feel of this monster's reiatsu pressing him down, beyond the sound of her gasped breathing somewhere behind him and to the right.

At full strength and in perfect health, perhaps this wouldn't be so onesided. But he is still injured from his earlier fight, wounds still tender, and he is tired. Impossibly exhausted in the way that not even a full night's sleep could cure. Maybe not even a week's worth.

The next assault surely would've ended his life, but Byakuya slips on the sand and slides into a kneeling position that effectively prevents his untimely end. However, the following flare of reiatsu is enough to steal his breath, to leave him panting with dark spots dancing before his eyes. The beast chuckles at that, knowing that the move was completely unintentional, realizing that his adversary is weak and wavering. He bounds in for the kill, exhilarated and radiating both contempt and murderous ecstasy.

And all Byakuya can think about as his vision tunnels in is the sound of her screaming at him. His name on her lips. The noise of her struggling to rise. To reach him in time. Already knowing that she won't make it, that if she manages to find her feet with so much power baring down on her it will be a miracle.

But just as a monstrous fist approaches, the beast is suddenly driven back by a rush of venomous yellow energy and cackling laughter. By a blur of sword swings and thrusts that are strong enough to draw blood. Byakuya would know this maniac anywhere, and he has never been so glad to see Zaraki Kenpachi in his entire life. Never been so thrilled to feel the stinging and blistering swell of his reiatsu as it blasts the Arrancar's back.

Byakuya sucks in air like a drowning man. He staggers to a standing position, and the words of a kidoh spell stream into his mind effortlessly. He casts without thought, charging and aiming on automatic. Whitish blue lightning lances the Espada square in the eyes, and he howls. Swinging violently. Stumbling as Zaraki takes the opportunity to all but cut out one of his legs from beneath him.

He isn't certain what happens next and will never be able to clearly recall. All Byakuya knows is that he hears her shriek, and then, he is flying. Crashing down to the sand and rolling to a stop beside the debris of Las Noches. He feels blood oozing from his chest to pool beneath him within seconds. A gash more than a scratch and likely the result of the monster's claws. Byakuya tries to stand, but the world unexpectedly does an odd whirl before his eyes. Swaying and spinning. His mouth tastes coppery, decidedly so, as he sinks back to the ground. Fingers digging into sand but finding no purchase. Doing little more than pull him down further.

He heaves in a breath, but it is still not enough to keep his head from swimming. To keep the entire universe from lurching and his belly from tying into knots. Not enough to allow him to crawl to his knees or to do more than merely watch as the Espada unleashes a cero directly towards him. Byakuya can only look on as the red flare of death stares him straight in the face and feel a momentary wash of sorrow that she has to see him die in such a manner. That she won't even have a body to bring back and bury.

However, the cero never reaches him. Never comes within ten feet. It is blown apart by a wave of pure black energy that rushes in from just beyond his shoulder. And there is an instant of silence after that. Of quiet realization that he is still alive, still breathing. Immensely relieved more for her sake than his own as his head lifts just enough to catch sight of his savior. Unable to distinguish him by reiatsu with so much being thrown around.

Orange hair. Black and flowing shihakushou. White hollow's mask.

Byakuya feels his brain short-circuit. All he can do is stare numbly from his position on the now red-soaked dunes. Practically gape as he watches another burst of black intercept the Arrancar's arm and send fluids flying.

Merciful gods in heaven… is that Kurosaki Ichigo?

Mask… the same mask he'd seen all those months ago. Latched onto the brat's – the boy's – face. But it is different this time. No internal battle or wild swings as he regains control. Now, he is deep waters with a blazing inferno within. Orange hair standing straight up, a stark contrast to the bone white mask on his face. But then, all of it becomes a streak as the boy takes off after the Espada. Moving at speeds that Byakuya couldn't even begin to contemplate at his level best. Much less now.

Much less as he feels his heart give a painful stutter. As he coughs and tastes copper on his lips. As his eyes shutter of their own accord, too heavy to remain open. As he hears more than sees someone stumble to his side. Which is followed by the sensation of little hands struggling to turn him over. Pressing to the wound on his chest with enough force to crack a rib. But it does little to stem the flow.

It takes him a minute, but Byakuya realizes that someone is speaking to him. Tone soft and urging. Pleading. Begging. Feels her reiatsu wash over him like a winter snow. Light and dainty and so refreshing that he can finally open his eyes.

She hovers above him like a vision. Eyes red-rimmed but so very blue. Tears staining her face.

"Nii-sama." Her voice is shaky, fingers trembling as they press against his skin. But she is as lovely as her older sister. "I'm here."

"I know," he tries to say. "I'm glad."

The words, however, are lost to a fit of coughing. To Byakuya choking on his own fluids. To the frantic feel of her trying to help him roll to the side to just breathe. He wishes to reassure her then, but all he can think of is the last time they were in this position. The memory of Shinsou's cutting edge and Aizen's cruel smile. And his fingers curl into fists, nails biting into his palms in a fit of self-revulsion. At the remembrance of his own depravity and the way she wept into his hand at the healing station.

"I didn't mean it," he wants to scream. "I take it back. All of it. I should've never let it get that far. Never should have allowed that to happen."

But he can't find the energy. Can't find his voice. The words slip from his tongue as her dainty hands tug at his shoulders. Holding on for dear life. Fingers pressing over the gaping hole in his chest as if by will alone she can hold back the blood.

"Just hold on. Just a little while longer, nii-sama," she begs. "Unohana-taichou will be here soon."

"I'm so proud of you," he wishes to reply. "Proud that you would risk so much to stand by your principles. By your friends. And I'm sorry that I could never tell you so before. Could never fully apologize for how badly I've failed you."

And something must show on his face because she reacts as though he spoke aloud.

"Thank you for saving me again. Thank you for coming, for helping us."

The words are like ash in his mouth. A taste both familiar and foreign and altogether bitter. He hasn't done nearly enough, anything at all really, and she thanks him like he has given her the world. And perhaps the last is what does it, what gives him the strength to speak in return.

He can feel her gaze on him, feel her fingertips brushing his hair back from his face.

"Nii-sama… I… Stay with me."

He wants to promise that he will, but Byakuya knows it is a lie. Knows that as surely as he comprehends the chill to his body and the slowing of his heart. Vision turning black even as he hears the faint echo of footsteps.

"Nii-sama," she whispers. Desperate and so full of hopeful sorrow. Of a hundred unnamed things that neither has ever dared to speak. "Just stay… A few minutes more. Look, she's here. Nii-sama…"

And the world fades to the white of a healing spell.

Imouto: little sister (something Byakuya has never called Rukia in the manga or anime to my knowledge)

Ever Hopeful,

Azar

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